FIG. 1 is a simplified side cross-sectional view of a portion of a field emission display 10 including a faceplate 20 and a baseplate 21 in accordance with the prior art. FIG. 1 is not drawn to scale. The faceplate 20 includes a transparent viewing screen 22, a transparent conductive layer 24 and a cathodoluminescent layer 26. The transparent viewing screen 22 supports the layers 24 and 26, acts as a viewing surface, and forms a hermetically sealed package between the viewing screen 22 and the baseplate 21. The viewing screen 22 may be formed from glass. The transparent conductive layer 24 may be formed from indium tin oxide. The cathodoluminescent layer 26 may be segmented into pixels yielding different colors to provide a color display 10. Materials useful as cathodoluminescent materials in the cathodoluminescent layer 26 include Y.sub.2 O.sub.3 :Eu (red, phosphor P-56), Y.sub.3 (Al, Ga).sub.5 O.sub.12 :Tb (green, phosphor P-53) and Y.sub.2 (SiO.sub.5):Ce (blue, phosphor P-47) available from Osram Sylvania of Towanda Pa. or from Nichia of Japan.
The baseplate 21 includes emitters 30 formed on a planar surface of a substrate 32. The substrate 32 is coated with a dielectric layer 34. In one embodiment, this is effected by deposition of silicon dioxide via a conventional TEOS process. The dielectric layer 34 is formed to have a thickness that is approximately equal to or just less than a height of the emitters 30. This thickness may be on the order of 0.4 microns, although greater or lesser thicknesses may be employed. A conductive extraction grid 38 is formed on the dielectric layer 34. The extraction grid 38 may be, for example, a thin layer of polysilicon. An opening 40 is created in the extraction grid 38 having a radius that is also approximately the separation of the extraction grid 38 from the tip of the emitter 30. The radius of the opening 40 may be about 0.4 microns, although larger or smaller openings 40 may also be employed.
Another dielectric layer 42 is formed on the extraction grid 38. A chemical isolation layer 44, such as titanium, is formed on the dielectric layer 42. A high atomic mass layer 46, such as tungsten, is formed on the chemical isolation layer 44 for reasons that will be explained below.
The baseplate 21 may also include a drive field effect transistor ("FET") 50 formed in or on the surface of the substrate 32 for controlling the supply of electrons to the emitter 30. The FET 50 includes an n-tank 52 formed in the surface of the substrate 32 beneath the emitter 30. The n-tank 52 serves as a drain for the FET 50. The n-tank may be formed by a conventional masking and ion implantation process. The FET 50 also includes a source 54 and a gate 56 separated from the substrate 32 by a gate dielectric 57 and a field oxide layer 58. The opening 40 in the high atomic mass layer 46 is typically about 10 microns in diameter, while the n-tank 52 is typically about 13 microns in diameter. The emitter 30 is typically about a micron tall, and several (e.g., four or five) emitters 30 are included together with each n-tank 52, although only one emitter 30 is illustrated.
The substrate 32 may be formed from p-type silicon material having an acceptor concentration N.sub.A ca. 1-5.times.10.sup.15 /cm.sup.3, while the n-tank 52 may have a surface donor concentration N.sub.D ca. 1-2.times.10.sup.16 /cm.sup.3.
In operation, the extraction grid 38 is biased to a voltage on the order of 100 volts, although higher or lower voltages may be used, while the substrate 32 is maintained at a voltage of about zero volts. Signals coupled to the gate 56 of the FET 50 turn the FET 50 on, allowing electrons to flow from the source 54 to the n-tank 52 and thus to the emitter 30. Intense electrical fields between the emitter 30 and the extraction grid 38 then cause emission of electrons from the emitter 30. A larger positive voltage, ranging up to as much as 5,000 volts or more but generally 2,500 volts or less, is applied to the faceplate 20 via the transparent conductive layer 24. The electrons emitted from the emitter 30 are accelerated to the faceplate 20 by this voltage and strike the cathodoluminescent layer 26. This causes light emission in selected areas, i.e., those areas adjacent to where the FETs 50 are conducting, and forms luminous images such as text, pictures and the like.
When the electrons strike the cathodoluminescent layer 26, they also cause soft X-rays to be emitted along with the visible photons. The high atomic mass layer 46 prevents other circuitry (not illustrated) that may be formed on the substrate 32, such as those that might drive the gate of the FET 50 from being exposed to penetrating radiation, such as X-rays generated from electron bombardment of the cathodoluminescent layer 26.
The high atomic mass layer 46 also may act as an electron lens, distorting the image created by the emitted electrons when it is too close to the emitter 30. Additionally, when the high atomic mass layer 46 is close to the extraction grid 38, particulate contamination can result in a short circuit between these two layers and cause catastrophic device failure. As a result, the edge of the opening 40 in the high atomic mass layer 46 is located above the middle of the gate 56 and the field oxide 58. At least some portions of the field oxide 58 cannot be shielded by the high atomic mass layer 46 from the X-rays and photons that are generated by the electrons striking the cathodoluminescent layer 26. Penetrating radiation, such as soft X-rays, gives rise to mobile charges, i.e., electron-hole pairs, when the radiation is incident on dielectric materials, such as gate dielectrics. The electrons that are generated are generally much more mobile than the accompanying holes. The electrons are thus able to respond to the high electric fields and migrate towards the n-tank 52 and emitter 30, leaving the holes trapped in the field oxide 58. The electrons at the interface influence the threshold voltage and transconductance of the FET 50 and may render the FET 50 totally inoperative. As negative charge builds up in the interface of the field oxide 58 and the silicon material, the drive FET 50 becomes progressively more leaky and eventually cannot be turned off.
Typical FET gate or field oxides can be made more robust by diffusing species such as fluorine and nitrogen into the gate or field oxides. These species act as recombination centers and/or as electron traps for radiation-induced charge carriers in oxides. However, the FETs 50 needed for active addressing of emitters 30 require that the field oxide 58 be generally much thicker than a typical field oxide, rendering this approach impractical or impossible.
What is needed is a way to render thicker oxides more robust to radiation damage for use in emitter drive FETs for field emission displays.